linux-kernel - Re: [PATCH 7/7] soundwire: intel: remove PDM support

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Date:   Mon, 10 Jan 2022 10:32:32 -0600
From:   Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
To:     "Liao, Bard" <bard.liao@...el.com>, Vinod Koul <vkoul@...nel.org>
Cc:     "alsa-devel@...a-project.org" <alsa-devel@...a-project.org>,
        "tiwai@...e.de" <tiwai@...e.de>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "broonie@...nel.org" <broonie@...nel.org>,
        "srinivas.kandagatla@...aro.org" <srinivas.kandagatla@...aro.org>,
        "Kale, Sanyog R" <sanyog.r.kale@...el.com>,
        Bard Liao <yung-chuan.liao@...ux.intel.com>
Subject: Re: [PATCH 7/7] soundwire: intel: remove PDM support

>>>>> While the hardware supports PDM streams, this capability has never
>>>>> been tested or enabled on any product, so this is dead-code. Let's
>>>>> remove all this.
>>>>
>>>> So no plans to test and enable this? Do the DMICs not use PDM?
>>>
>>> The point is that this code is unused/untested. We can re-add it after
>>> it was tested.
>>
>> That does not answer my question. Do the DMICs not use PDM?
> 
> I will leave the question to Pierre. 😊

traditional digital microphones using the Nokia spec with two
microphones using opposite clock edges do use PDM.

PDM over SoundWire is possible as well, it's e.g. used by Qualcomm for
amplifiers. This patch does not make a statement on whether PDM support
makes sense or not in general.

As mentioned above on Intel platforms the hardware is in theory capable
of supporting PDM, but that capability has never been deployed, all
existing devices we connect to are PCM based and all the PDM references
are essentially untested - and likely broken.